1. Field of the Invention
The present invention relates to an optical apparatus used in producing, for example, semiconductor elements or liquid crystal display elements by a photolithography process. More particularly, the invention relates to a catadioptric projection exposure apparatus using a catadioptric projection optical system suitably applicable as a projection optical system for 1:1 projection or for demagnification projection while attaining a resolution of submicron order in the ultraviolet wavelength region.
2. Related Background Art
A projection exposure apparatus is used in producing semiconductor elements or liquid crystal display elements by a photolithography process. The projection exposure apparatus demagnifies, for example, at a ratio of about 5:1 a pattern formed on a photomask or reticle (hereinafter generally referred to as "reticle") through a projection optical system to transfer a demagnified image of the pattern onto a substrate (wafer, glass plate or the like) coated with a photosensitive material (photoresist or the like). With an increase in degree of integration for semiconductor elements, a demand becomes vivid to attain a projection optical system with a higher resolving power for use in a projection exposure apparatus.
To satisfy the demand, the wavelength of illumination light must be shortened, or the numerical aperture (NA) of projection optical system must be increased. However, as the wavelength of illumination light becomes shorter, influence of optical absorption increases so as to limit types of glass materials which can be practically used. Particularly, with wavelength below 300 nm, practically usable glass materials are only synthetic quartz and fluorite. In this case, the two materials have respective Abbe's numbers which are too close to each other to correct the chromatic aberration. Thus, correction of chromatic aberration is extremely difficult if the projection optical system is constituted only by a refracting optical system. Also, the fluorite has bad change characteristics of index of refraction with a change in temperature. That is, it has large changes in temperature characteristics. Further, the fluorite has various problems in machining, e.g., in lens polishing. Because of such problems, the fluorite cannot be used for many parts in the projection optical system. Consequently, it is very difficult to enhance the resolution by the projection optical system constituted only by the refracting optical system.
On the other hand, an attempt has been made to construct a projection optical system only of a reflection optical system, because the reflection optical system causes no chromatic aberration. In this case, the projection optical system must be large in scale and have an aspherical reflection surface. It is also very difficult to constitute the projection optical system only by the reflection optical system.
Then, there are various techniques proposed to construct a reduction projection optical system of a so-called catadioptric optical system which is an optical system in a combination of a reflection optical system and a refracting optical system. An example is the reduction projection exposure apparatus provided with a ring field optical system, as disclosed in Japanese Laid-open Patent Application No. 63-163319. The ring field optical system is so arranged that exposure is carried out by the slit scan exposure method only for an image of a pattern portion in an off-axial annular zone, using an off-axial beam without causing interference between incident light and reflected light.
Another example is a reduction projection exposure apparatus provided with a projection optical system in which a beam splitter composed of a semitransparent mirror is set, as disclosed in Japanese Patent Publication No. 51-27116, Japanese Laid-open Patent Application No. 2-66510, U.S. Patent No. 4953960 or U.S. Pat. No. 5052763. The projection optical system is so arranged that an image of a pattern is projected by the full exposure method using a near-axial beam.